Large-scale folding of rocks during the process of mountain building is characteristic of metamorphism.

In the metamorphism, the rock is altered with the effects of natural agencies. Regional Metamorphism is the type of metamorphism, where the rocks are also changes with the interaction of high pressure or heat on the open wide area instead of contact with the magma. These rocks are also changes with the overburden of plate tectonics. In this question, the large-scale folding of rocks represents Regional Metamorphism.

The given blank can be filled with regional.

Explanation:

Regional metamorphism takes place over a much greater region. This metamorphism generates rocks like schist and gneiss. Regional metamorphism is the result of large geographic procedures like mountain building.  

These rocks when getting exposed to the surface demonstrate the astounding pressure, which makes the rocks to get dissociated or bent due to the phenomenon of mountain building. The regional metamorphism generally gives rise to foliated rocks like schist and gneiss.  

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.

"d.) carbon dioxide and hydrogen sulfide"